(1) Field of the Invention
This invention relates generally to trophic or growth factors and, more particularly, to the novel growth factor, neurturin.
(2) Description of the Related Art
The development and maintenance of tissues in complex organisms requires precise control over the processes of cell proliferation, differentiation, survival and function. A major mechanism whereby these processes are controlled is through the actions of polypeptides known as “growth factors”. These structurally diverse molecules act through specific cell surface receptors to produce these actions.
In recent years it has become apparent that growth factors fall into classes, i.e. families or superfamilies based upon the similarities in their amino acid sequences. Examples of such families that have been identified include the fibroblast growth factor family, the neurotrophin family and the transforming growth factor-beta (TGF-β) family.
Of particular importance are those growth factors, termed “neurotrophic factors”, that promote the differentiation, growth and survival of neurons and reside in the nervous system or in innervated tissues. Nerve growth factor (NGF) was the first neurotrophic factor to be identified and characterized (Levi-Montalcini et al., J. Exp. Zool. 116:321, 1951 which is incorporated by reference). NGF exists as a non-covalently bound homodimer. This factor promotes the survival and growth of sympathetic, neural crest-derived sensory, and basal forebrain cholinergic neurons. In sympathetic neurons this substance produces neurite outgrowth in vitro and increased axonal and dendritic growth in vivo. Early indications as to the physiological roles of NGF were obtained from in viva studies involving the administration of neutralizing antibodies (Levi-Montalcini and Booker, Proc Nat'l Acad Sci 46:384–391, 1960; Johnson et al. Science 210: 916–918, 1980 which are incorporated by reference), and these studies have been confirmed by analyzing transgenic mice lacking NGF via gene targeting (Crowley et al., Cell 76:1001–12, 1994 which is incorporated by reference). NGF has effects on cognition and neuronal plasticity, and can promote the survival of neurons that have suffered damage due to a variety of mechanical, chemical, viral, and immunological insults (Snider and Johnson, Ann Neurol 26:489–506, 1989; Hefti, J Neurobiol 25:1418–35, 1994 which are incorporated by reference). NGF also is known to extensively interact with the endocrine system and in immune and inflammatory processes. (Reviewed in Scully and Otten, Cell Biol Int 19:459–469, 1995; Otten and Gadient, Int. J. Devl Neurosci 13:147–151, 1995 which are incorporated by reference). For example, NGF promotes the survival of mast cells. (Horigome et al. J Biol Chem 269:2695–2707, 1994 which is incorporated by reference).
It became apparent that NGF was the prototype of a family of neurotrophic factors upon the discovery and cloning of brain-derived neurotrophic factor (BDNF) (Liebrock et al. Nature 341:149–152, 1989 which is incorporated by reference), which was the second member of this family to be discovered. The relationship of BDNF to NGF, is evidenced in the conservation of all six cysteines that form the three internal disulfides of the NGF monomer (Barde, Prog Growth Factor Res 2:237–248, 1990 which is incorporated by reference). By utilizing the information provided by BDNF of the highly conserved portions of two factors, additional members (NT-3, NT-4/5) of this neurotrophin family were rapidly found by several groups (Klein, FASEB J 8:738–44, 1994 which is incorporated by reference). Information concerning their distribution and activities, and the physiologic consequences of their deficiencies (via gene targeting), has greatly increased our knowledge of neuronal development (for reviews, see Jelsma et al., Curr Opin Neurobiol 4:717–25, 1995; Lindsay et al., Trends Neurosci 17:182–90, 1994; and Johnson et al., Curr Biol 4:662–5, 1994 which are incorporated by reference). For instance, it is now clear that the various neurotrophins act on largely non-overlapping neuronal populations (e.g. motor neurons, sub-populations of sensory neurons), and regulate their survival and metabolism in ways similar to those originally described for NGF. Their identification has also led to refinements in the neurotrophic hypothesis, as evidence has accumulated that neurons can switch their neurotrophin survival requirements during maturation (for review, see Davies, Curr Biol 4:273–6, 1994 which is incorporated by reference).
Recently, the understanding of the mechanisms of signal transduction for neurotrophic factors has been advanced by the identification of receptors for the NGF family of neurotrophic factors. The tyrosine kinase receptor, trkA, identified as the NGF receptor and the closely related receptors trkB, which mediates signaling of BDNF and NT-4/5, and trkC, which mediates effects of NT-3, have allowed dissection of the signal transduction pathways utilized by these neurotrophins (for review, see (Tuszynski et al., Ann Neurol 35:S9–S12, 1994 which is incorporated by reference). Signaling by NGF involves proteins which interact directly with the phosphorylated trkA receptor (e.g. Shc, PLCγ1, PI-3 kinase), other trkA substrates like SNT (Rabin et al., Mol Cell Biol 13:2203–13, 1995 which is incorporated by reference), and downstream kinase effectors (e.g. ras, raf1, MEK and MAP kinase). In some cases, particular components have been linked to specific actions of NGF, such as Shc and PLCγ1 requirement for neurite outgrowth (Loeb et al., J Biol Chem 269:8901–10, 1994; Stephens et al., Neuron 12:691–705, 1994 which is incorporated by reference) and PI-3 kinase requirement for survival (Yao and Cooper, Science 267:2003–6, 1995 which is incorporated by reference).
In addition to the discovery of molecules related to NGF, structurally unrelated neurotrophic factors have also been recently identified. These include factors originally isolated based upon a “neurotrophic action” such as ciliary neurotrophic factor (CNTF) (Lin et al., Science 246:1023–5, 1989 which is incorporated by reference) along with others originally isolated as a result of non-neuronal activities (e.g. fibroblast growth factors (Cheng and Mattson Neuron 1:1031–41,1991 which is incorporated by reference), IGF-I (Kanje et al, Brain Res 486:396–398, 1989 which is incorporated by reference) leukemia inhibitory factor (Kotzbauer et al, Neuron 12:763–773, 1994 which is incorporated by reference).
Glial-derived neurotrophic factor (GDNF), is one such neurotrophic factor structurally unrelated to NGF. GDNF was, thus, a unique factor, which, up until now, was not known to be a member of any subfamily of factors. The discovery, purification and cloning of GDNF resulted from a search for factors crucial to the survival of midbrain dopaminergic neurons, which degenerate in Parkinson's disease. GDNF was purified from rat B49 glial cell conditioned media (Lin et al., Science 260:1130–2, 1993 which is incorporated by reference). Sequence analysis revealed it to be a distant member of the superfamily of transforming growth factor β (TGF-β) factors, having approximately 20% identity based primarily on the characteristic alignment of the 7 cysteine residues (Lin et al., Science 260:1130–2, 1993 which is incorporated by reference). Thus, GDNF could possibly have represented a new subfamily within the TGF-β superfamily.
GDNF, like other members of the TGF-β superfamily, has a precursor molecule, with a signal sequence and variably sized pro-region, that is generally cleaved at an RXXR site to release the 134 amino acid mature protein, GDNF. Thus, GDNF is synthesized as a precursor protein.
Subsequent processing results in a mature glycosylated homodimer of approximately 35–40 kD. Six of the seven cysteines form intrachain disulfide bonds and connect hydrogen-bonded β-sheets to make a rigid structure called a cystine knot (McDonald et al., Cell 73:421–4, 1993 which is incorporated by reference), a structure which, interestingly, is also characteristic of the neurotrophins. The remaining cysteine forms a disulfide bond with another monomer to form the biologically active hetero- and homodimers. This structure may account for the strong resistance of GDNF to denaturants such as sodium dodecyl sulfate (SDS), heat and pH extremes.
Recombinant GDNF produced in bacteria specifically promotes the survival and morphological differentiation of dopaminergic neurons in midbrain neuronal cultures (Lin et al., Science 260:1130–2, 1993 which is incorporated by reference). These initial in vitro experiments have now been extended to in vivo models which demonstrate that GDNF has potent protective and regenerative effects on MPTP- or axotomy-induced lesions of dopaminergic neurons in adult rodent brain (Tomac et al., Nature 373:335–9, 1995 and Beck et al., Nature 373:339–41, 1995 which is incorporated by reference). GDNF promotes the survival in vitro of nodose sensory and parasympathetic neurons, and can rescue chicken sympathetic neurons from NGF deprivation-induced death, but this requires much higher doses than are necessary for its effects on dopaminergic neurons (Ebendal et al., J Neurosci Res 40:276–84, 1995 which is incorporated by reference). Significantly, GDNF is retrogradely transported by motor neurons and is known to promote the survival of motor neurons inasmuch as animals treated with GDNF suffer much less motor neuron loss in response to lesions than untreated animals or those treated with other trophic factors such as CNTF, BDNF, NT-3 or NT-4/5 (Henderson et al., Science 266:1062–4, 1994; Yan et al., Nature 373:341–4, 1995; and Oppenheim et al., Nature 373:344–6, 1995 which are incorporated by reference). Overall, GDNF was a more potent factor for promoting the survival of motor neurons than the other factors, and it was the only factor that prevented neuronal atrophy in response to these lesions, thereby positioning it as a promising therapeutic agent for motor neuron diseases.
Neuronal degeneration and death occur during development, during senescence, and as a consequence of pathological events throughout life. It is now generally believed that neurotrophic factors regulate many aspects of neuronal function, including survival and development in fetal life, and structural integrity and plasticity in adulthood. Since both acute nervous system injuries as well as chronic neurodegenerative diseases are characterized by structural damage and, possibly, by disease-induced apoptosis, it is likely that neurotrophic factors play some role in these afflictions. Indeed, a considerable body of evidence suggests that neurotrophic factors may be valuable therapeutic agents for treatment of these neurodegenerative conditions, which are perhaps the most socially and economically destructive diseases now afflicting our society. Nevertheless, because different neurotrophic factors can act preferentially through different receptors and on different neuronal cell types, there remains a continuing need for the identification of new members of neurotrophic factor families for use in the diagnosis and treatment of a variety of acute and chronic diseases of the nervous system.